[PATCH] Mitigate PR85222 a bit

[Jonathan Wakely]

On 6 April 2018 at 12:48, Jonathan Wakely wrote:
> Currently --with-default-libstdcxx-abi doesn't alter the ABI of the
> shared lib when the dual-abi is enabled, just whether the initial
> value of the macro is 0 or 1. All the symbols in the library, and the
> type of exception thrown, are the same for every dual-abi build of the
> library.

I mean every dual-abi build for a given release, of course. Because
the type of exception thrown changed between gcc 6 and gcc 7, so maybe
my position that the option doesn't affect the ABI of the .so is
untenable, given that it changed independent of that option :-\